1. Field of the Invention
The present invention relates to an optical scanning device and, more particularly, to an optical scanning device for use in a laser-beam printer in which light beams emitted from a plurality of light sources are directed to a deflection surface of deflection means at a predetermined angle thereto in a sub scan longitudinal section and a plurality of beams reflected and thus deflected by the deflection means are used to optically scan a recording medium (a photoconductor drum) and to record image information.
2. Description of the Related Art
A variety of multi-beam optical scanning devices have been proposed which present a multi-color recording by introducing a plurality of light beams to a common scanning optical system. In such a type of multi-beam optical scanning device, to independently project a plurality of light beams to a surface being scanned, the light beams need to be reflected and thus deflected by an optical deflector (a polygon mirror, for example) and then need to be split. To this end, however, spatial separation is required for the same wavelength light sources. For example, spatial separation is made possible by introducing the light beams to the deflection surface (reflection surface) of the optical deflector at an oblique angle in a sub scan longitudinal section.
In a magnifying optical scanning device with its optical system under the pressure of compact size demand, the optical path for spatial separation permitted is short, and an angle of incidence to the deflection surface becomes large, thereby presenting the following two major problems.
A first problem is the scanning line curvature on a surface being scanned, and a second problem is image quality degradation. These problems will now be discussed with reference to the optical system shown in FIGS. 1A and 1B.
FIGS. 1A and 1B are sectional views of a major portion of the optical system of a conventional magnifying optical scanning device, in which light beams are obliquely introduced. FIG. 1A is a sectional view of a main scan and FIG. 1B is a sectional view of a sub scan perpendicular to FIG. 1A. As shown, two light beams obliquely incident on the deflection surface of an optical deflector 21 are reflected and deflected.
The optical deflector 21 is a polygon mirror, for example. Two obliquely incident light beams 22 are reflected and deflected by the polygon mirror. Image forming means 32 comprises two-element f.theta. lens system having a cylindrical lens 23 with a predetermined refracting power in a main scan plane or a longitudinal section and two-stacked toric lenses 24. The cylindrical lens 23 gets the lens backward space of the f.theta. lens system 32, contributing to shifting the entire lens system closer to the polygon mirror 21 and thus making the optical system compact. As shown in FIG. 1B, the two-stacked toric lenses 24 comprise, in the direction of the sub scan, upper and lower toric lenses 24a, 24b, which are independently arranged in the respective optical axes of the two obliquely incident light beams 22.
The operation of a light incident system for introducing two light beams at an oblique angle to the optical deflector 21, though not shown, is as follows: a plurality of light beams emitted from a plurality of light sources and corresponding to the two obliquely incident light beams 22 are collimated into parallel light beams through a collimator lens, and are then brought to a focus as a line in the vicinity of the deflection surface 21a of the polygon mirror 21 through a cylindrical lens that has a predetermined refracting power in the sub scan longitudinal section only. This serves as means for correcting the tilt of the deflection surface of the polygon mirror in the sub scan longitudinal section or plane, making the deflection surface of the polygon mirror optically conjugate to a surface being scanned (the surface of a photoconductor drum) with respect to the sub scan longitudinal section, and namely, constitutes a tilt correction optical system.
The plurality of light beams 22 (obliquely incident beams) reflected from the mirror 21 are guided to predetermined positions on the photoconductor drum through unshown optical path bending mirrors by the f.theta. lens system 32. With the polygon mirror 21 rotating, the scanning lines are drawn in the direction of the main scan in which the light beams are deflected while the photoconductor drum rotates in synchronization with the polygon mirror 21 so that the scanning lines are formed at regular intervals in the sub scan direction perpendicular to the main scan direction. In this way, by simultaneously projecting two light beams onto the photoconductor drum, two-color development is made possible during a single revolution of the photoconductor drum, expediting color printing.
The oblique incidence of light has been conventionally unused in the magnifying optical system, because the following problem arises when the light beams are obliquely directed to the deflection surface 21a of the polygon mirror 21 in the sub scan longitudinal section.
The problem of the oblique incidence of light is discussed referring to the light beam (obliquely incident light rays) reflected downward with respect to the axis of symmetry X of the toric lenses 24a, 24b shown in FIG. 1B.
The first problem occurs when the light beam obliquely incident on the deflection surface 21a of the polygon mirror 21 scans along a conical surface as shown in FIG. 2A in the sub scan longitudinal section (a plane perpendicular to X-Y plane). The light beam deviated as above enters the toric lens 24a arranged in the optical axis of the obliquely incident light beam 22, and then draws a scanning line curved due to such deviation on the surface of the photoconductor drum as the surface being scanned. This is the scanning curvature problem of the oblique angle of incidence. In the magnifying optical system, the obliquely incident angle Q needs to be as large as 3.degree. through 6.degree., and this translates to the amount of curvature of the scanning line in the range of 0.5 mm to several mm on the photoconductor drum, which is sufficiently large.
A solution to this problem is a plane-parallel glass plate (a correction glass) arranged in the optical axis. However, this correction arrangement needs a thick glass plate making the apparatus bulky and pushing the cost of the apparatus.
The second problem is the image quality degradation. FIGS. 2B, 2C and 2D illustrate how the image quality degrades. FIG. 2B is an explanatory view of light beams incident on the deflection surface 21a of the polygon mirror 21 in the plane of the main scan. As shown, P is a principal beam and U and L are light beams on both sides of the principal beam P. FIG. 2C shows the sub scan longitudinal section or sub scan plane of FIG. 2B.
As shown, light beams L, P and U are reflected as light beams having different heights in that order in view of the sub scan plane. Since the light beams are brought to a focus as a line image in the sub scan direction by the incident tilt correction cylindrical lens (not shown), the light beams L, P, and U are expressed as a rotated line image having an angle of rotation of .phi. relative to the main scan plane (X-Y plane) on the deflection surface of the polygon mirror as shown in FIG. 2D. The angle of rotation of .phi. varies in proportion to the angle of rotation of the polygon mirror.
FIG. 3A is an explanatory view showing the inclination of the light beams in the sub scan longitudinal section on the deflection surface of the polygon mirror, and FIG. 3B is an explanatory view showing the orbit of the principal light beam P which is caused to scan by the polygon mirror and the light beams U and L on both sides of the principal light beam P. Meridional beams are the ones, referenced to the X-Y plane, defined by the entrance optical system for projecting the incident beams onto the polygon mirror.
As shown in FIGS. 3A and 3B, as the angle of view in the main scan plane (an absolute value of the Y coordinate) increases, the angles of rotation .phi. of light beams L, P and U increase. The light beams L and U rotate about the principal beam P in the Y-Z plane, and are affected by refracting power in the meridional direction of the toric lens 24a. Thus, the image quality degrades.
FIG. 4 illustrates how the image quality degrades as the angle of rotation of .phi. of the light beam increases with an increase of the angle of view of main scan. A spot image is increasingly distorted to a star-like configuration as the main scan angle of view increases.